A typical such tap changer has at least one fine selector for connection of a plurality of taps of the control winding, and also at least one preselector that is connected with the primary winding and by which the control winding is connectable in a different polarity with the primary winding. Through the series connection that is effected by means of the tap changer, of the primary winding with the control winding it is possible to tap desired different voltages in correspondence with the tapping of the control winding. It is possible in this way to freely select the input/output voltage of the control transformer in a desired range. A tap changer of that kind is needed for each phase of the three-phase current. The tap changer is usually constructed as a rotary switch. The preselector and fine selector can be constructed to be integrated on the axle of the rotary switch. The fine selector is usually formed by fine selector contacts arranged on different planes of a rotary switch. On each plane, each second tap of the control winding is respectively connected in alternation. Switching takes place between the fine selector contacts, which are arranged on the two planes, by a load changeover switch that is known per se and usually includes switching resistances. An arrangement of that kind is described in, for example, DE 10 2009 060 132 [U.S. Pat. No. 8,576,038]. However, the fine selector can also be formed by a fine selector contact that connects all contacts, which are arranged on a plane, of the control winding. This fine selector contact can also comprise switching resistances.
There is now a problem with, in particular, switching the preselector, i.e. if the polarity of the control winding has to be changed by means of the preselector. In that case, the connection of the preselector with a first winding end of the control winding is switched to a connection with the second winding end of the control winding. This switching process is, in fact, usually carried out in current-free state, yet voltages are induced not only in the primary winding, but also in the control winding and these voltages can lead, together with the capacitive coupling of the windings with one another and with earth, to an arc and thus to generation of gas during switching of the preselector.